After an early career with the Royal Shakespeare Company (1951–54) and several critically acclaimed appearances in plays by Eugene O'Neill, Bannen turned his focus toward film roles. Since the mid-1950s, he has appeared in numerous British and American films, most notably, The Flight of the Phoenix (1965) for which he received an Oscar nomination, Gandhi (1981), Damage (1992), Braveheart (1995), Waking Ned Devine (1998) and To Walk with Lions (1999). He was killed in a car crash near Loch Ness, Scotland.